1. Field of the Invention
The present invention relates to an access method for an optical disk drive and, more particularly, to a precise and high speed access method involving a multi-track or long-range jump to reach a target track.
2. Description of the Related Art
An optical disk drive is usually accessed in steps: first, a coarse access step and then a fine access step. For the coarse access step, a stepper motor or a voice coil motor (VCM) is generally used to move a carriage on which an optical head is mounted. However, positioning the optical head by a stepper motor is not precise and it is difficult to control the velocity of a VCM in order to reach a target track precisely. Therefore, the fine access step is needed to reach the target track using an actuator installed in the optical head to perform a multi-track jump. After the target track is pulled in, a track servo mechanism for the track actuator holds the target track during subsequent operation.
FIG. 1 illustrates schematically the structure of an optical disk drive, especially the relationship between an optical disk 10 and an optical head assembly 12. The disk 10 is rotated by a spindle motor 11. The optical head assembly 12 is loaded on a carriage 14 which is moved by a VCM or stepper motor 16. A light beam 28 from a laser diode 20 is collimated by a collimator lens 22 before passing through a beam splitter 24 and is focused on the optical disk 10 by an object lens 23 mounted on an actuator 18. When the optical disk drive receives a command to access a target track, the VCM or stepper motor 16 moves carriage 14 in a radial direction relative to disk 10 (as indicated by arrow 30) toward the target track in response to a control signal from a controller (not shown). After the carriage 14 has moved an instructed distance, a servo mechanism incorporated in the optical disk drive is switched on and an instantaneous track is pulled in. A photo detector 26, receives reflected light from the optical disk 10 via the beam splitter 24, detects the present track position and the deviation from the target track is calculated. When the track position is found to be within the range of fine access, coarse access is switched over to fine access.
FIG. 2 is an exploded perspective view of actuator 18. The object lens 23 is fixed on member 32, which is rotatable back and forth around axis 34 by a tracking coil 36 attached thereto and also movable up and down axially by a focus coil 38. The light beam 28 comes in from the right side of FIG. 2 and is reflected toward the object lens 23 by a mirror 35. The beam path is more complex than that shown in FIG. 1 because of the actual structure of actuator 18. The actuator or access head 40, including object lens 23, member 32 and coils 36 and 38, is inserted in magnet subassembly 42, which comprises magnet pole pieces 44 and a support yoke 46. A detector 39 is provided to detect deviation from the neutral position of actuator head 40. A light source for detector 39, such as a light emitting diode (not shown), is fixed inside window 41. One function of actuator 18 is to move the focused beam 28 radially on the optical disk 10 during fine access, and another function, after fine access is completed, is to make the focused beam 28 follow the center of the target track and to maintain precise focusing of the light beam during read/write operations of the optical disk. The above second function includes a servo operation to compensate for shifting of the beam position inwardly or outwardly due to decentering of the optical disk 10. The second function also includes moving the object lens 23 in an axial direction to maintain good focus during servo operations. Therefore, this type of actuator 18 is called a two-dimensional actuator.
There are many ways in the prior art to move the actuator head 40 in fine access. A single-track jump method (also called a micro jump method) is the most fundamental and simplest method. A pair of accelerating and decelerating current pulses is applied to the tracking coil 36 of FIG. 2 in order to move the focused beam 28 by one track pitch. Each positive pulse accelerates the actuator head 40 during a first half cycle and then the subsequent negative pulse decelerates the head 40 during the following half cycle. This single-track jump is repeated, as illustrated in FIG. 3(a), a specified number of times to reach a specified track.
The focused beam 28 moves radially on the optical disk surface by one track pitch in response to each pair of pulses. This process is followed repeatedly while the required number of tracks are counted. Photo detector 26 of FIG. 1 generates the track error signal shown in FIG. 3(b), which is responsive to the relative position of the focused beam on the chasing track of optical disk 10, and is utilized to adjust the beam position. Generally the track error signal has a sinusoidal waveform where one period corresponds to a movement of the focused beam across one track pitch between two adjacent tracks. The above single-track jump method has a demerit in that a relatively long time is required to reach the target track.
To shorten the access time, multi-track jump methods have been introduced. The methods all utilize a basically similar idea. The principle is explained using FIGS. 4 and 5. FIG. 4 shows a schematic block diagram of a system using a multitrack jump method and FIG. 5 shows the signal waveforms related to the operation of FIG. 4.
When the fine access step begins at time T0, a controller 50, comprising a microcomputer, memory circuit, etc., sends a jump command to jump pulse generator 52 and, at the same time, an open command to track servo circuit 54, both being connected to tracking coil 36 in actuator head 40. A jump pulse, such as the rectangular waveform shown in FIG. 5(a), having a duration of T is applied. The jump pulse accelerates actuator head 40 with a specified force for duration T. The rotational velocity of actuator head 40 is increased and then it is maintained at a constant speed from time T1 to T2 as shown in FIG. 5(b). At time T2, a pulse voltage of reverse polarity having the same absolute height H is applied to tracking coil 36 for duration T, and this decelerates the actuator head 40 and brings it to a standstill at time T3.
At that moment (T3), the track servo circuit 54 is again activated as shown in FIG. 5(d) by a command in a servo control signal from controller 50 and the target track is pulled in. During the access operation, a track error signal shown in FIG. 5(c) is generated in detector 26 and supplied to counter circuit 58. Counter circuit 58 counts the zero crossing points of the track error signal and is cooperatively connected to controller 50 which calculates the number of tracks to the target track. FIG. 5 shows a case in which six tracks are jumped over.
In applying a multi-track jump method, there are two problems to be solved. The first problem is decentering of the optical disk rotation and the second is a spring force effect which influences the movement of actuator head 40 of FIG. 2. The rotating optical disk has a deviation of rotation radius reaching up to 50 to 60 .mu.m. This deviation is caused by a lack of mechanical precision of the optical disk itself during its production and further by a lack of precision in installing the optical disk on a spindle motor. The above deviation value is much larger than the track pitch such as 1.6 .mu.m which is generally used. During the movement of the actuator head in fine access, the optical disk rotates forward by a certain angle causing a displacement of the focused beam far away from the estimated track due to decentering and this causes a failure in fine access.
As for the second problem, actuator head 40 of FIG. 2 is installed on magnet subassembly 42 and is to be rotated freely around axle 43 by activation of tracking coil 36 and moved axially by focus coil 38. However, these two coils 36, 38 are connected to outside fixed terminals by connecting wires (not shown), which supply driving current. These connecting wires exert spring forces on the actuator head 40. This spring force tends to pull back the actuator head 40 toward a neutral position during tracking and the force increases with the rotation angle of actuator head 40. Therefore, it is necessary to compensate for this force by adding a driving current to the tracking coil 36 to avoid tracking failure.
Several patents have been disclosed to solve the above problems. Japanese Patent Tokukaisho (Laid Open Patent) 61-22479 dated Jan. 31, 1986 discloses a compensation method for decentering of an optical disk in a multi-track jump using this method. Japanese Patent Tokukaisho 62-54835 dated Mar. 10, 1987 discloses that the duration of accelerating and decelerating jump pulses can be adjusted by measuring the velocity of the actuator. Japanese Patent Tokukaisho 60-205836 dated Oct. 17, 1985 discloses a compensation method for the spring force using a gradually increasing compensating current supplied to the tracking coil during a multi-track jump; however, this patent does not deal with the decentering of an optical disk. Japanese Patent Tokukaisho 61-230630 dated Oct. 14, 1986 discloses a compensation method for both the spring force and the decentering of an optical disk; however, compensation of the decentering of an optical disk is difficult in a long-range jump using this method.
In the above described methods, the estimated track error is compensated gradually during a whole jump range; therefore, if the multi-track jump is especially long, such as more than 100 tracks, it is difficult to access the target track without a failure when both decentering of optical disk and spring force should be compensated.
A completely different technique is disclosed in "Seek Techniques for the Optotech 5984 Optical Disk Drive" by Everett Bates et al., SPIE Vol. 695, Optical Mass Data Storage, 1986. The technique disclosed in the Bates et al. article is summarized below with reference to FIGS. 6(a) and 6(b). FIG. 6(a) shows a track error signal after the initial acceleration jump pulse. The track error signal is similar to the curve shown in FIG. 5(c). When it is assumed that the velocity of the actuator head 40 after an acceleration jump requires To, e.g., 100 microseconds, to cross one-half track pitch, each duration of Ta to Tc, Tc to Te, Te to Tg, etc., of FIG. 6(a) should be 100 microseconds ideally. In an actual case, the time required to travel one-half pitch deviates in both directions from the expected value T.sub.o. The Bates et al. article discloses that every time the track error signal crosses zero, a deceleration pulse is applied, the pulse having a width of approximately half the duration of T.sub.o, in this case, 50 microseconds. And then, during the remaining time, T.sub.o -50 microseconds, an acceleration pulse having the same amplitude is applied. This is shown in FIG. 6(b). All deceleration pulses between Ta and Tb, Tc and Td, Te and Tf, and Tg and Th have the same pulse width and height. In FIG. 6(b), the pulse width of Tb to Tc is 50 microseconds, the pulse widths of Td to Te, and Tf to Tg are longer than 50 microseconds and the width of Th to Ti is shorter than 50 microseconds. As a result, when the optical head moves faster than expected, it is decelerated. And on the contrary, when it moves slower than expected, it is accelerated. Thus the travel time to cross one-half track pitch is adjusted to maintain the travel time almost constant throughout fine access. However, this method has a weak point when the track error signal includes a noise pulse. When the noise pulse crosses zero level, it works to generate another decelerating pulse successively; therefore, the actuator is accelerated or decelerated too much.